1) Field of the Invention
The present invention relates to the improvement of the cycle characteristics of non-aqueous electrolyte secondary cells and batteries.
2) Description of the Related Art
Non-aqueous electrolyte secondary cells represented by lithium ion secondary cells have a high energy density and a high capacity, and as such are useful for the power sources for driving mobile information terminals. As the mobile information terminals have become more and more multifunctional, such cells are required to have still higher capacity.
As the positive electrode active material for the non-aqueous electrolyte secondary cells, lithium cobalt oxide (LiCoO2) is often used for its high cell capacity and excellent charge and discharge characteristics. However, when used alone, the lithium cobalt oxide may not show sufficient thermal stability and cycle characteristics. In view of this there have been proposed techniques of adding to the lithium cobalt oxide different metal elements such as Ti, Zr, Mg, and Al. Such techniques are described in Patent Documents 1 to 6.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-200605 (Abstract)
Patent Document 2: Japanese Unexamined Patent Application Publication No. 6-325791 (pages 2 to 3)
Patent Document 3: Japanese Unexamined Patent Application Publication No. 4-319260 (Abstract)
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2002-208401 (Abstract)
Patent Document 5: Japanese Unexamined Patent Application Publication No. 6-168722 (Abstract)
Patent Document 6: Japanese Unexamined Patent Application Publication No. 2001-68167 (Abstract)
i) Patent Document 1 discloses the use of a non-aqueous electrolyte containing propylene carbonate and LiN(SO2C2F5) 2 as the electrolyte salt, a graphite negative electrode, and LiCoO2 containing Ti as the positive electrode material in which titanium particles and/or titanium compound particles are attached on the surfaces of the lithium cobalt oxide particles. The mole ratio of the titanium particles and/or titanium compound particles to the lithium cobalt oxide is 0.00001 to 0.02. Here, the titanium particles serve to decompose a coating film that results from the non-aqueous solvent (in such a manner that the film encompasses the positive electrode active material), or serve to promote the removal of the coating film. This inhibits the deterioration of discharge characteristics resulting from faulty ion conductivity. As a result, this document claims that a significant drop in the discharge capacity at the time of operation under low temperatures is alleviated.
ii) Patent Document 2 discloses a positive electrode active material that is mainly composed of first-particle-agglomerations (second particles) having an average diameter of 0.1 μm to 15 μm. The first-particle-agglomerations are composed of first particles having an average diameter of 0.01 μm to 5.0 μm. Also, the positive electrode active material comprises LixMy1Ny2O2 (M representing a Co, Ni, or V atom; N representing a Ni, V, Fe, Mn, Ti, B, or P atom; x=0.1 to 1.5, y1=0.8 to 1.4, y2=0 to 0.5, and z=1.90 to 4.2). It is claimed that by controlling the particle diameters the resulting non-aqueous secondary cell is provided with preferable application characteristics and preferable charge and discharge characteristics, and further, preferable self-discharge characteristics are provided.
iii) Patent Document 3 discloses the use of a lithium cobalt oxide to which zirconium is added. Here, the surfaces of the lithium cobalt oxide particles are covered with zirconium oxide or a compound oxide of the lithium and zirconium, and thus are stabilized. The document claims that this eliminates the decomposition reaction of the electrolyte and crystal defects even at a high potential, thus realizing excellent cycle characteristics and excellent preservation characteristics.
iv) Patent Document 4 discloses the use of a positive electrode active material that is composed of a lithium-containing transition metal compound oxide. This oxide is substantially represented by the formula LixTyMzO2 (where T represents at least one element selected from the transition metals; M represents at least one element selected from the group consisting of Mg, Al, Si, Ti, Zn, Zr, and Sn; 0.9≦x≦1.15, 0.85≦y≦1.00, and 0<z≦0.1). Here, the particle diameters can be made fine by calcination under the normal conditions. As a result, this document claims to realize an excellent cell capacity, excellent charge and discharge characteristics, and excellent temperature characteristics (especially those of low-temperature), in accordance with the fine particle diameters, the sphericalness of the particles, and sharpness of particle distribution.
v) Patent Document 5 discloses the use of, as the positive electrode active material, LiMgxCo1-xO2-y (0<x<1, 0<y<0.5, and x=2y). Compared with LiCoO2, this substance excels in electron conductivity at normal temperature, thus improving the cell performance.
vi) Patent Document 6 discloses a non-aqueous electrolyte cell in which a power generating element composed of a positive and a negative electrodes and the electrolyte are housed in an outer casing. The outer casing can be deformed by only a slight increase in the internal pressure, and the electrolyte is one of gelled polymer in which a solid polymer, an electrolyte salt, and an electrolytic solution are gelled. Further, the positive electrode active material is a lithium-containing compound oxide represented by the formula LiCo1-xZrxO2 (0<x≦0.1). Such a structure inhibits the decomposition of the solvent and electrolyte salt, and thus inhibits gas generation within the cell. As a result, it is claimed that cell dilation is hard to occur even though the outer casing is susceptible to a slight increase in the internal pressure.
When, in accordance with the above techniques, a lithium cobalt oxide is used as the positive electrode active material to which an element different from cobalt is added, the electrolyte can be decomposed through the charge and discharge cycles and the amount thereof is decreased. In addition, the products resulting from the decomposition cause to increase the internal resistance, thus aggravating the cycle characteristics. This aggravation caused by the decomposed electrolyte becomes especially notable when the amount of the active material is increased and that of the electrolyte is decreased, in the hope of enhancing the cell capacity. Thus, the cell capacity cannot be enhanced sufficiently without compromising the cycle characteristics.